An Investigation of Fatigue Origins in Superalloy Powder Compacts

In axial ly loaded smooth fatigue spec imens at room and elevated t empera tu re , there is a g rea te r p robabi l i ty of having a surface initiated fatigue fa i lu re than subsurface crack initiation. The less cons t ra ined su r face provides a zone of re la t ive ly easy local ized sl ip. This , coupled with minor sur face flaws, such as, machining and handling marks , co r ros ion pits, and oxidized a reas , i n c r e a s e s the probabi l i ty of surface r e lated crack ini t ia t ion. Fo r these r easons , when subsurface fatigue c rack in i t ia t ion s i tes a re observed on the f r ac tu re surface , the cause for these in i t ia t ions is of pa r t i cu l a r concern . It indicates that the m a t e r i a l in that a r ea contains s t r e s s r a i s e r s , such as, defects or cer ta in m i c r o s t r u c t u r a l e lements , that make the subsurface locat ion more suscept ib le to crack ini t ia t ion. In many cases subsur face in i t ia t ion is assoc ia ted with fatigue life degradat ion 1 due to p r e m a t u r e fatigue cracking. The re fo re , the ident i f ica t ion and the e l i m i n a t ion of these e l ements r espons ib le for the subsur face ini t ia t ion has the potential of improving the fatigue life of the ma te r i a l . A p rec i s ion sec t ioning technique was developed 2 to pe rmi t the meta l lographic examina t ion and chemica l ana lys i s of the m i c r o s t r u c t u r a l e l ements su r round ing the in i t ia t ion s i te . Th i s method was success fu l ly used to de te rmine the causes of subsur face fatigue c rack ini t ia t ion of ce r ta in t i tan ium alloy wrought products , 3 cast ings 4 and powder compac tsP ,6 In the cases of t i t an ium alloy powder compacts , poros i ty and nonmeta l l i c inc lus ions were shown to be r e spons ib le for s u b s u r face fatigue c rack in i t ia t ion. It was the object ive of this work to inves t iga te the fatigue or ig ins of supe ra l loy powder compacts , to see if the p r ec i s i on sec t ioning technique could also be used to analyze the i r natu re . The two alloys that were se lec ted for this inves t iga t ion are AF-115 and AF2-1DA. The major a l loying e l emen t s of each a re l is ted in Table I. Both al loys a re powder me ta l lu rgy al loys designed to operate at e levated t e m p e r a t u r e s and were produced by an argon a tomiza t ion p rocess .7 Tes t spec imens were fatigued in s t r a i n control at 760~ (1400~ in l abora tory a i r . Fat igue tes ts were conducted only under tens i le s t r a i n s . The f rac tu re su r faces were well p r e se rved , and the fatigue c rack in i t i a t ion s i tes were eas i ly ident i f iable . In all the examined cases , where the fatigue c rack in i t ia t ion si te was t raced to a subsur face location, the p rec i s ion sec t ion ing ana lys i s r e vealed that the or ig in of the fa i lure was e i ther at a pore or nonmeta l l i c inclus ion. This is very s i m i l a r to the na ture of the subsur face fatigue c rack or ig ins found in the t i t an ium alloy powder compactsP '6